This invention relates to a method and apparatus for using frequency modulated laser beams for optical communication and more particularly to a method and apparatus for such optical communication through cross-connected channels involving multiple laser frequencies.
The goal of optical communications is to exploit the extremely large bandwidth of the optical portion of the electromagnetic spectrum in ways analogous to traditional electrical communications. The overall performance of coherent optical communication systems, or the "Figure of Merit", is specified as the product of the amount of information transmitted per second and the distance of the transmission path. In order to improve the Figure of Merit from the current level on the order of 0.5 Terabit km/sec (0.5.times.10.sup.12 bits km/sec), it is necessary not only to improve the individual technologies in the various relevant fields, such as those related to transmitters and modulations techniques, but also to determine an optimum mixture of those technologies.
With respect to transmitters, laser diodes such as solid state laser diodes have been considered because they can be directly modulated faster than is currently possible with external cavity modulators. They can also provide peak optical power at wave lengths that correspond to minimums in the fiber optics' absorption profile, they can be mass-produced, and they require little support circuitry. On the other hand, however, there is a major drawback associated with laser diodes when they are modulated by amplitude shift keying (ASK) in that output light pulses vary both in intensity and in frequency. When a light pulse propagates down the fiber, the frequency components spread out at a rate that depends on both the modulation rate and the quality of the fiber. Future improvements in this technology are not expected to significantly reduce this problem.
With respect to the quality of optical fibers, the uniformity of the glass used in the fibers is approaching the practical economic limit as defined by the current technology. Neither is there expected a large improvement in total system performance due to improved fiber quality.
With respect to modulation techniques, ASK is not the only means for encoding data. Coherent optical signals can be made to interfere with each other in a way directly analogous to frequency modulated (FM) radio signals. One may thus modulate either the relative phase or the frequency of the optical signals (collectively referred to as "angle modulation", for which homodyne and heterodyne methods are available for detection). Although it is not practical to directly angle-modulate a laser beam, both these detection techniques have superior noise immunity relative to ASK.
When one considers optical communications, it is also necessary to take into account the scattering effects of coherent light in an optical fiber caused by the interaction of the light with an acoustic pressure wave generated within the fiber. Although the system performance can generally be improved by increasing the input optical power, it also has the effect of increasing the strength of the generated acoustic wave. The severity of the scattering, however, depends on the method used to modulate the input light signal. The use of angle modulation greatly suppresses the threshold for stimulated scattering as compared to when ASK modulation is used.
A further problem which should be considered relates to wavelength division multiplexing (WDM), which means the use of more than one light signal in an optical communications system, more particularly to the interaction of multiple optical signals within a fiber. When multiple optical signals are combined onto one fiber, stimulated Raman scattering can pump optical power out of the lower wavelength channels into higher wavelength channels as an example of "optical cross talk". The cross talk is present either with ASK or with angle modulation and represents a fundamental limitation on both the number of optical signals that can be combined onto a fiber and the optical power than can be contained in each signal.